Class-D amplifiers amplify an audio signal for driving a loudspeaker or other dynamic load using efficient switch-mode power conversion techniques. Such switching amplifier topologies typically have a transistorized output stage capable of generating a high voltage, high speed square wave that is passed through a low-pass filter to reduce the high frequency component and effectively provide an analog reconstruction filtration process. The low pass filter is typically terminated by the load attached to the amplifier, which broadly comprises various loudspeakers and/or similar transducers. These loads are traditionally designed to present a nominal impedance to the output filter, typically in the range of 2 to 16 ohms.
As with any amplifier, negative feedback is used to correct errors induced through the various stages of signal modulation, amplification, filtering, and reconstruction. Unfortunately, closing the feedback loop around all the stages within a Class-D switching amplifier and maintaining stability over a broad range of load and drive conditions can be difficult. This is especially true given the phase shift aspects of the passive output filter (LC low pass) when operating with various real-world loading scenarios.
The task of closing the feedback loop is further complicated by the fact that many Class-D amplifier modulation schemes have non-fixed switching frequencies. In other words, the switching frequency changes with the modulation index, as the amplifier output signal swings toward a higher drive amplitude, the switching frequency decreases, and as the amplifier output signal moves toward the zero crossing or lower amplitude, the switching frequency increases. This dynamic modulation of the switching frequency complicates the task of ensuring stability over all load scenarios.
Previous amplifier designs have focused on traditional loop stability solutions in which appropriate pole zero placement within a combination of proportional, integral, and differential stages results in marginal phase margin over a limited range of operability and modulation index. Unfortunately, there are cases where instability can occur due to non-linear load dynamics, light load or open load conditions, high slew rate stimulus, and excessive switching frequency modulation. When this instability is exposed, these amplifiers can suffer from sustained high frequency oscillation, excessive power draw, increased thermal dissipation, very poor high-modulation or clipping performance, and can ultimately result in output stage component failure.
It should also be noted that previous feedback compensation and loop filtration schemes have utilized fixed pole-zero placement in which the transfer function of the compensation elements and the loop filter has been fixed. In this case, the pole-zero positioning within the compensation network and the loop filter have a fixed transfer function and have been tuned for the worst-case phase shifts caused by various elements within the loop, including the dynamic phase shift associated with the passive output filter. Some designs have utilized one global feedback loop, whereas others have attempted two feedback loops, but in all cases the pole-zero placement in the compensation and loop filter stages are fixed and have a fixed transfer function. Thus, the loop transfer function cannot be adjusted or modulated in real-time and can only be configured for one operational condition, which is typically the worst-case phase shift scenario resulting from an unterminated output filter. Unfortunately, these previous designs do not allow for improved feedback operation under different operational conditions and loads, and therefore, the audio performance is less than ideal.
Additional problems have also arisen in multi-channel Class-D audio amplifier designs with non-fixed switching frequencies. The changing asynchronous switching frequencies of closely spaced Class-D modulators has resulted in issues such as beat notes, increased noise floor, and excessive channel-to-channel cross-talk or inter-channel interference. While the root cause has been understood to be the dynamic and non-fixed nature of the Class-D modulator frequencies, fixing the switching frequencies of the closely spaced modulators can be a difficult task, especially over a wide portion of the modulation index range.
Previous attempts to fix the switching frequency of Class-D modulators have generally suffered from limited range or increased complexity and cost. For example, some designs have opted for hysteretic window modulation to attempt fixed frequency operation. Unfortunately, this technique typically does not work over the entire modulation index desired from the amplifier and is limited solely to hysteretic implementations. Other prior art approaches have focused on carrier injection techniques, which offers simplified implementation, but again only provide a limited range of fixed frequency operation, wherein the switching frequency only remains fixed below a certain modulation index. In other words, as the amplifier output swings to higher levels there is an amplitude above which the switching frequency will no longer remain constant. As the amplitude is increased further, the Class-D modulator switching frequency will move further away from the intended fixed frequency operation. This deviation in switching frequency below the intended operating frequency will result in greatly increased output ripple, reduced open loop bandwidth and loop gain, and can lead to increased distortion within the output waveform.
Alternatively, other schemes have attempted to fix the switching frequency through complex frequency locking loops that effectively modulate the loop gain of the Class-D control loop. These approaches do offer a wider range of fixed frequency operation, but the frequency locked loop topologies have a high degree of complexity, require voltage-controlled gain stages and PID control structures, and a higher cost associated with implementation.
Another element of Class-D amplifiers that presents various problems to a designer is the passive output filter. The output low pass filters on Class-D amplifiers are typically constructed with passive LC components which are optimized for termination into nominal loudspeaker load impedances. Unfortunately, the impedance of real-world loudspeakers is rarely a fixed impedance and in many cases the loudspeaker load can be disconnected entirely, leaving the output LC filter without an audio-band termination impedance. Previous designs have attempted to minimize these problems through Zobel RC terminations that offer some termination impedance at high frequencies. Unfortunately, these terminations are lossy, generate excessive heat, add to system cost, and many times do not terminate the filter adequately during open-load scenarios.
Additionally, if a designer wishes to encompass the output filter within one or more feedback loops, the wide range of terminating impedances and the resulting varied phase shifts wreak havoc on maintaining loop stability. For example, a passive LC output filter when terminated by the designed termination impedance will exhibit a nominal phase shift of 90 degrees, which can be accounted for within the closed loop, post-filter feedback compensation. However, when the same passive LC filter is terminated with non-ideal load impedances or open load conditions, the nominal phase shift will increase to 180 degrees and exhibit very sharp asymptotic change at the natural resonant frequency of the LC output filter. With this poor termination scenario, the closed loop feedback compensation has difficulty maintain adequate phase margin and can lead to undampened oscillation. Also, if the loop compensation elements are adjusted to try maintaining stability during open load scenarios, the loop performance is not ideally tuned for situations wherein the output filter is properly terminated, such as when a loudspeaker termination is present, thus the amplifier's distortion will be elevated due to poorly optimized loop gain. In other words, designs that attempt to include the output filter within a control feedback network face great challenges due to the dynamic and changing phase characteristics of the output LC filter. Optimizing for stability during open load situations, leads to poor distortion performance under normal load situations, conversely, optimizing for distortion performance under normal load scenarios leads to poor phase margin, instability, and potential for oscillation under open load conditions.
As a result of the aforementioned problems with the state of the art in Class-D audio amplifiers, what is needed is a Class-D amplifier design that provides a fixed frequency architecture with a feedback control methodology that is stable over all load and drive scenarios.
The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.